The present invention relates to a method of determining an indicator of a signal transit time in a digital circuit, to a method of regulating the operating voltage of a digital circuit, and to a circuit arrangement for regulating the operating voltage of a digital circuit.
The performance of a digital circuit is determined to a critical degree by the transit time of signals in its circuit elements. If the signals in the digital circuit can pass through the circuit elements in a short transit time, or in other words, quickly, then the circuit can be operated at a higher frequency and, hence, with a higher performance. In particular, what determines performance in this case is the transit time for the longest signal path, also termed the critical path. The number of critical paths in a digital circuit may range from one to several thousand, the latter being the case in for example circuits which process a large number of bits in parallel and/or have many pipeline stages. The maximum operating frequency for synchronous, clocked digital circuits is then equal to the reciprocal of the transit time for the longest signal path.
Due to temperature-related effects, voltage fluctuations and production-related factors, the signal transit time on the critical signal path is not precisely known. Thus, a safety margin is normally introduced. This safety margin lays down a maximum permitted operating frequency equal to the reciprocal of a maximum transit time, where the maximum transit time is the sum of the signal transit time on the critical path and of other maximum increases in transit time which may arise from the temperature variations or voltage fluctuations or production-related factors that can be expected. In this way, the increase in transit time due to the effect of temperature is determined by the temperature range specified, which may extend from −20° C. to +100° C. for example. The increase in transit time due to the supply voltage represents the effect of the sporadic variation in the supply voltage. If the supply voltage is varied, the signal transit times alter, with a higher voltage giving shorter transit times and a lower voltage longer ones.
By using a suitable detection circuit for the digital circuit and for the current temperature to determine the transit time on the critical path, it is possible to make use of the dependence which signal transit time has on the operating voltage to set the maximum transit time. The effects caused by temperature and process fluctuations on transit time on the critical path are compensated for in this way. If, for example, the circuit is operated at ambient temperature and the circuit elements of the digital circuit have a short signal transit time, then even quite a low supply voltage will be enough to allow the required frequency to be achieved. The power consumption of the circuit is thus lower, which is desirable particularly in portable applications.
To allow a digital circuit to be operated with as low an operating voltage as possible, it is known for the digital circuit to have assigned to it an auxiliary circuit providing a simulation or so called replica of a critical path. If the signal transit time on this critical path exceeds a given value, the supply voltage is raised. If the signal transit time drops below a set value, it is lowered. The particular disadvantage that this known method has is that if there are major fluctuations in the transit times in the individual circuit elements there may be too great a difference between the signal transit time on the replica of the critical path and the signal transit time in the digital circuit, which means that regulating the operating voltage by reference to the signal transit time in the replica may give rise to unacceptably high signal transit times in the digital circuit. To counter this, provision may be made for an increasing safety margin as the fluctuation in the signal transit times in the circuit elements increases, but this has the disadvantageous consequence that an unnecessarily high operating voltage may be set in certain circumstances. When this is done, the statistical fluctuations in gate transit time increase for semiconductor structures of the same dimensions, and it is thus precisely with modern-day CMOS technologies and high-performance circuits, where the operating frequency may be more than 1 GHz, that the operating voltage can, at best, only be regulated by the known method by accepting the disadvantage of an excessively high operating voltage.